Evolution appears to favour women having children at a younger age.
Photograph: JGI/Tom Grill/Getty Images/Blend Images

As humans continue to evolve, natural selection appears to be favouring higher body mass index (BMI) in men and an earlier age for starting a family in women, research has revealed.

Researchers used data from the UK Biobank, a large genetic and health database of half a million British people aged 45 and over, to look at how numerous traits from body mass index to height and birth weight, as well as particular genetic variations associated with such traits, are linked to the number of children individuals had during their lifespan.

But scientists note that the effects are weak and that it will take many generations before significant changes are seen in humans. In addition, they stress it is not clear if natural selection is acting directly or indirectly on the traits.

“We wanted to try to understand what types, and [to] quantify, the evolutionary forces that are affecting contemporary human traits, including height and BMI,” said Jaleal Sanjak, a co-author of the research from the University of California, Irvine.

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“And also characterise whether selection was pushing the population in one direction or another, or favouring intermediate values or extreme values,” he added.

It also found that while studies have shown natural selection has disfavoured both very heavy and very light babies, that selection is now extremely weak, and only seen in females.

“The strength of natural selection is a fraction of what it was, and this birth weight thing is a beautiful example of that because now neonatal care is so good that you can be very underweight or very overweight and it makes no difference,” said Steve Jones, emeritus professor of human genetics at University College London, who was not involved in the study.

“The interesting thing is not that there are bits of natural selection around – which without question there are. But what is called the opportunity for natural selection has almost disappeared.”

In men, the team found that natural selection appears to favour a higher body mass index. But with obesity linked to problems with fertility, said Jones, “higher BMI suggests that big muscly hunks are the ones who do better”.

However Sanjak said there might be more to the findings, pointing out that it is difficult to unpick cause and effect.

“The problem here is that the genetic variants that predispose an individual to have a higher BMI also seem to predispose an individual to have more children, that is true. But it could be the case that having more kids has an effect on your BMI.”

He added that it could be that BMI was genetically linked to other traits that were under natural selection.

The study, he added, also suggested that natural selection is favouring reduced educational attainment in women.

But, it seems, that is not a straightforward link, with further analysis revealing that the result is most likely linked to selection on the age of starting a family, with younger mothers less likely to have reached higher levels of education, or less able to reach them once having had children.

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The team also showed that starting a family earlier in life appears to be favoured by natural selection in women.

“We understand that having kids earlier should mean you have more of them, but the surprising part was just to observe it on the genetic level,” Sanjak said.

Overall, he said, the results paint an interesting picture. “Natural selection is still happening in modern humans – it is observable, we can detect it.

“But they are fairly weak effects and secular trends, things due to modern medicine and social change, are likely to be bigger drivers of changes in these traits.”

For example, while natural selection was favouring reduced educational attainment, educational attainment overall was increasing over time. “That is an example of a secular force that is going to swamp the effect of natural selection,” he said.